rbgs will always be both more competitive and stimulating for smarter players, those who are willing to apply knowledge of game mechanics to changes in the environment aka people who can think quickly in more than one dimension will come out on top.
let us for example imagine a situation in which two mice are placed in a box, and that they have to battle to the death. Clearly, the more physically capable, bloodthirsty mouse would come out on top and skill wouldn't be a factor [this is an analogy for playing a better comp, for all you arena people reading this].
RBGs definitely have a higher skillcap, there is so much more to do as almost every class, and the strategies can be much more complex than arenas. Unfortunately majority of the best players in the game no longer participate in them. They are far less competitive now than previous seasons, and the best teams are very mediocre compared to what they could be. The main reason people who struggle in arenas tend to excel more in RBGs aren't because they're easier, its because the teams beating them in arena don't exist in RBGs, they choose not to play them. The player pool is much much smaller.
Anyone who has actively played both RBGs and arenas at the highest level would agree its much more fun and satisfying to win a hard fought RBG with two amazing teams than to win any arena match, but nowadays that is INCREDIBLY hard to come by, it may not even exist anymore.